The focus of the proposed experiments is to investigate cell signaling mechanisms between cell layers during organogenesis. Our previous work has shown that the columnar cells and the peripodial ceils in imaginal discs of Drosophila both grow in precise proportion to each other, suggesting coordinated regulation by a global size control mechanism. The first specific aim of this proposal is to provide a molecular description of the signaling pathways between the two cell layers. Studies with Drosophila imaginal discs are advantageous because upon identification of a pathway, we can remove or activate components of the pathway. This will tell us whether the pathway is only used for size control or organ identity or both. Based on preliminary data we focus our study on the wg, dpp, hh, and insulin signaling pathways between the two cell layers. In Drosophila, just as in other organisms, upon the wounding of an organ, wound healing occurs. A cut through an imaginal disc brings two cell layers together, and the two cell layers fuse shortly afterwards. The second aim of the proposal examines whether the molecular signals identified in the first specific aim also function in regeneration after wound healing. Preliminary observations indicate that wingless induces fate map changes necessary for regeneration. An understanding of how molecular signaling increases developmental potency during regeneration is key for stem cell biology.